Tuesday, 11 December 2012

integration - Showing the limit of intixnfty(ylogy)1dy is zero



I'm trying to show



lim



In order to finish a proof. The problem I'm having is that without the limit, I know the integral diverges, and hence when I use substitution I end up with indeterminate form.



I think rather than using a substitution like v = \log x, I need to re-write the integral in the form of e^{-t} so that the integral can be expressed in a proper form.




Any hints/advice is appreciated thank you!



Edit:



Using the substitution t=y \log y I end up at an integral of the form \int \frac{1}{1+e^t}dt which does not seem to help as I again end up with an indeterminate form


Answer



\int \limits_{x}^{\infty} \frac{1}{y \ln y} dy > \int \limits_{x}^{x^2} \frac{1}{y \ln y} dy = \ln \ln x^2 -\ln \ln x = \ln 2 \approx 0.693 > 0 for all 0 < x \in \mathbb{R}, so if \lim \limits_{x \to \infty} \int \limits_{x}^{\infty } \frac{1}{y \ln y} dy = 0 then we would have that for all x > x_0 that \int \limits_{ x}^{x^2} \frac{1}{y \ln y }d y < \epsilon which contradict the fact that \int \limits_{x}^{x^2} \frac{1}{y \ln y} dy = \ln \ln x^2 -\ln \ln x = \ln 2 \approx 0.693 and \epsilon = 0.1


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